A PIFA antenna apparatus (Planar Inverted F-Antenna) is known from JP 11-150415. The antenna apparatus described there having an inner antenna surface, which is in the form of a passive element, and an outer antenna element, which is approximately annular, has the disadvantage that it can be operated at only two different resonant frequencies.
Furthermore, an antenna apparatus is known from JP 2000-68736 which, on the basis of a ground surface, has two or more antenna surface branches, which each operate at individual resonant frequencies. The branching antenna surface is equipped with a single radio-frequency connection.
A PIFA antenna apparatus in the form of a so-called stacked PIFA antenna, is likewise known from the prior art, namely from EP 0 871 238 A2, but this requires a considerable volume, which is disadvantageous for use in the mobile radio field, particularly for installation in a cellular telephone.
In order to reduce the volume of a PIFA antenna, U.S. Pat. No. 6,195,048 B1 proposes the provision of both an inner antenna surface and an outer antenna surface surrounding the inner antenna surface, both of which are connected to a radio-frequency connection and have a common groundpoint. An antenna such as this cannot be operated as a tri-band antenna since the two antenna surfaces that are provided can be operated, respectively in only a single resonant band.
An antenna for mobile communications terminals should comply with both technical and visual requirements. Firstly, it should operate for more than one frequency while, secondly, it should be as small as possible in order, for example, to allow it to be integrated in a mobile radio in such a way that it cannot be seen from the outside. By way of example, a PIFA antenna, which has an extremely compact form, may be used for this purpose.
These PIFA antennas generally operate in two independent frequency bands; for example, in the 900 MHz frequency band for GSM operation (GSM=Global System for Mobile Communication) and in the 1800 MHz frequency band for PCN operation (PCN=Personal Communications Network). The introduction of a further, third frequency, such as in the frequency band for PCS operation (PCS=Personal Communications Services) of 1900 MHz, has until now been implemented mainly with the aid of the refinements described in the following text.
One known refinement for introducing a third frequency provides for the bandwidth of the PCN frequency band of a dual-band antenna to be widened by increasing the distance between the antenna and the printed circuit board (PCB) of the mobile radio. This allows the GSM/PCN dual-band antenna to be used for the PCS frequency band as well.
However, in this refinement, in order to operate in three frequency bands, the PIFA antenna requires approximately 50% more volume than a PIFA antenna which operates in only two frequency bands.
Another refinement for designing a PIFA antenna which operates in three frequency bands provides for a dual-band PIFA antenna to be combined with an additional antenna. This additional antenna provides the third resonant frequency, and the entire PIFA antenna apparatus now has three frequencies. In this case, both the dual-band PIFA antenna and the additional antenna are connected to an RF input/output of the mobile radio, with the two antennas being driven separately. This refinement once again leads to the PIFA antenna apparatus being relatively voluminous.
Since present-day mobile radios are becoming ever smaller, these PIFA antenna apparatuses which have been described so far, and which are designed for three frequency bands, are not suitable for installation in mobile radios such as these.
An object of the present invention is, thus, to develop a PIFA antenna apparatus for three frequency bands, whose volume is not significantly greater than existing dual-band PIFA antennas, thus allowing it to be installed in existing mobile radios.